A couple of years back I ran into a couple who were running for Express-1. They had a 2004 Sprinter 2500 that looked showroom ... with around 400,000 miles on it.
Their secret ?
They kept it waxed (Turtle Wax ICE if I recall correctly)
I didn't keep ours waxed and have paid the price on the outer body surfaces ...
I'd agree that the Mercedes outer body paint and corrosion protection surely ain't the greatest ... but OTOH, my chassis and undercarriage is in great shape ... despite being driven in all seasons, all over the US.
Had I kept it waxed I'd bet the outer body would look a whole lot better than it does now.